1. Field of the Invention
This invention relates to iron alloy strips having so high a magnetic flux density and corrosion resistance that they are suited for use as yokes to construct magnetic circuits of voice coil motors in magnetic recording equipment. It also relates to yokes for voice coil motor magnetic circuits.
2. Description of the Related Art
In general, a hard disk unit includes a medium having a magnetic recording film, a spindle motor for rotating the medium at a predetermined rotational speed, a magnetic head for writing and reading information data, a voice coil motor (VCM) for driving the magnetic head, a control device and the like. The voice coil motor has a magnetic circuit which is constructed by a permanent magnet for generating a magnetic flux and yokes combined therewith and used as an actuator for driving the head. In the magnetic circuit for CD and DVD drives, a permanent magnet for generating a magnetic flux and yokes combined therewith are used to construct an actuator for driving a pickup lens. The current drastic competition among manufacturers imposes the requirement of a further cost reduction on voice coil motors.
The first priority for parts used in VCM is cleanness or no dusting. Yokes and other iron parts which are liable to rust are generally used after surface treatment for imparting corrosion resistance because rust releases contaminant particles with which heads and lenses of hard disk and pickup units are contaminated. Additionally, parts themselves are fabricated in a clean manufacture procedure, which inevitably increases the cost of parts. Nevertheless, strict cleanness management is needed to avoid crushes between the magnetic head and the medium and contamination of lenses.
As the yokes in magnetic circuits that constitute voice coil motors, inexpensive customary rolled steel strips of SPCC, SPCD and SPCE are often used to meet the requirement of cost reduction. These customary rolled steel strips are characterized by ease of working such as blanking and bending and a low cost, but fail to inhibit rusting. In a common practice taken in the art to solve the rust problem, after steel strips are worked by a press machine or the like, expensive electroless Ni—P plating is carried out for rust prevention.
To achieve a cost reduction of magnetic circuits, inexpensive materials such as SPCC have been used as described above. Since customary rolled steel strips are unlikely to be corrosion resistant, expensive corrosion resistant metal coatings as by nickel plating must be formed. The inevitable result is a cost increase.
As discussed above, cold rolled steel strips such as SPCC are most often used because of improved productivity as by blanking, shaping, piercing, bending and embossing, and a low cost. However, these steel strips, due to the lack of satisfactory saturation magnetization and corrosion resistance, are difficult to avoid magnetic saturation in partial VCM magnetic circuit when made to a small size and thin wall, failing to fully carry the magnetic flux from a permanent magnet having a high magnetic flux density to the magnetic circuit. The gage of yokes is limited by the restrictions associated with the overall apparatus, which fails to effectively utilize all the magnetic flux of the high performance magnet, leading to partial saturation or magnetic flux leakage midway the magnetic circuit.
The magnetic flux leakage not only reduces the magnetic flux density across the gap of the magnetic circuit, but also has an impact on the adjacent magnetic recording medium and control unit. A certain prescribed limit is imposed on the quantity of magnetic flux leakage from the VCM circuit and the quantity of magnetic flux leakage from products must be below the prescribed value.
Also, to avoid particle contamination such as rust, a surface treatment film must be formed, which makes it quite difficult to reduce the cost.
It would be quite desirable to have a yoke-forming magnetic material which can eliminate the magnetic flux leakage, make full use of a high magnetic flux density inherent to a permanent magnet, and be manufactured at a low cost.